Combined Effects of Misonidazole Radiosensitization and Hypoxic Cell Toxicity in Mammalian Cells

It has been established that the hypoxic cell radiosensitizer misonidazole (Ro-07-0582) is also a toxic agent to hypoxic cells in the absence of radiation. In this study, the combined effects of radiation sensitization and toxicity were examined in hypoxic CHO cells. Complete radiation survival curves were measured for cells in the presence of 15 mM misonidazole prior to or following incubation with the drug for various times in hypoxia at 37°C. The latter case is the condition for the "preincubation effect" observed previously for misonidazole. The cells were irradiated as a monolayer in stoppered glass flasks with 280-kVp X rays; survival was assayed by colony formation. The survival data obtained in these experiments were analyzed by comparing the experimentally observed survival curves with those theoretically predicted based on the concepts of independent and additive interaction of the two effects of hypoxic cell toxicity and radiation sensitization. For incubation times of 1-3 hr, for which the cytotoxic effect is moderate, the combined response appears to be of an additive rather than an independent nature. The data in fact suggest a somewhat greater than additive response, and also a slight change in the slope as well as the shoulder of the survival curve, when compared with the predicted curves. The results are discussed with regard to both in vitro and in vivo studies.